Spectroscopic analytical instruments are widely used for analysis of samples. As used in herein, the term “spectroscopic analytical instrument” refers to dispersive infrared spectrophotometers (“IR”), Fourier transform infrared (“FTIR”) spectrophotometers, fixed band path filtrometers, ultra violet and visible light spectrophotometers (“UV/VIS”), near infrared spectrophotometers (“NIR”), Raman spectrophotometers and any other device used for spectroscopic analysis in which the absorbance by a sample of light energy emitted by the spectroscopic instrument is detected by such instrument. Spectroscopists and other technicians performing analysis by transmission sampling normally place the sample in some type of optically pure sample holding device which positions the sample in the path of the light energy beam emitted by the spectroscopic analytical instrument and allows the energy to be transmitted through the sample without being blocked or absorbed by the sample holder. The instrument's detector then detects the amount of energy absorbed by the sample throughout the region of the spectrum which is within the instrument's range.
When a spectroscopic analytical instrument detects the energy absorbed by the sample, the absorbance is typically represented graphically as vertical peaks shown along the ordinate (y axis) of a graph on which the spectral range is shown as the abscissa (x axis), although absorbance can be represented digitally. Digital readouts are more common for inexpensive instruments such as fixed band pass filtrometers. It is desirable to have a nonabsorbing sample holder so that spurious absorbances from the sample holder do not interfere with the analysis.
IR and FTIR spectrophotometers normally employ sample holders containing high quality crystal optics as sampling substrates, while UV/VIS, Raman and NIR spectrophotometers use sample holders comprised of silica and certain types of silica glass and fused silica compositions. The distinguishing feature of the desirable light energy transmitting materials used in all spectroscopic analytical instruments is that they do not absorb energy in all or a significant part of the spectral region where the instrument operates.
Sample holders using high quality crystal optics which have been shaped and polished by precision opticians for optimum performance are capable of transmitting enough infrared energy in the spectral region of interest to the infrared spectroscopist to enable performance of both qualitative and quantitative analysis of a sample. When performing quantitative analysis by transmission sampling, the infrared spectroscopist can define the thickness of the sample by sandwiching the sample between two crystal windows separated by a spacer. Many infrared spectroscopists also prefer to sandwich the sample between windows when performing qualitative analysis.
In cases where the analysis involves studying the absorbance peaks of a sample which occur in a limited spectral region, the spectroscopist can sometimes tolerate use of a sample holder which absorbs energy in another spectral region as the absorbance peaks created by the sample supporting substrate will not obscure the analysis. It is also possible to compensate to a certain extent for absorbance peaks created by the sample holder by either placing a duplicate sample holder in the compensating beam of a double beam IR spectrophotometer or by running a background with an FTIR spectrophotometer which subtracts or ratios the background from the scan of the sample. Taking a background also compensates for water and CO2 in the atmosphere and for low energy transmission through the sample supporting substrate. But where absorbance peaks are strong, use of backgrounds to ratio those absorbance peaks out of the analysis of the sample becomes an ineffective technique. Rather than to rely upon backgrounds as compensating tools, many infrared spectroscopists prefer to use optically pure sampling supporting windows with high energy throughput and to purge the sample compartment of atmospheric contamination (typically H2O and CO2) using a dry gas such as nitrogen.
A business unit of Minnesota Mining & Manufacturing Corporation (“3M”) introduced a polymer based IR sample card (U.S. Pat. No. 5,764,355) several years ago for use in transmission testing which was available with two sample supporting windows viz. polyethylene & polytetrafluoroethylene (“PTFE”). More recently, another company, Spectra-Tech, has begun manufacturing sample cards with polyethylene and PTFE sampling support windows. Unfortunately, when a spectroscopist does an analysis of a sample applied to a polyethylene or PTFE window, an analysis is being made of both the sample and the window. Both of these sample cards use sampling substrate windows that have strong absorbances in the spectral range of interest for most IR and FTIR spectrophotometers and for other spectroscopic analytical instruments. As shown in FIG. 1, the window in the PTFE card exhibits strong absorbance peaks in the important 1300 to 450 cm−1 region (in particular at 1223.6 cm−1, 1156.1 cm−1, 639.4 cm−1, 554.7 cm−1, and 502.9 cm−1). As shown in FIG. 2, the window in the polyethylene card exhibits strong absorbance peaks at 2918.7 cm−1 and 2849.9 cm−1 and somewhat less pronounced but still annoying peaks at 1473.1 cm−1, 1462.9 cm−1, 730.2 cm−1 and 719.9 cm−1. As noted above, by running a background, certain absorbance peaks can be ratioed or subtracted, but they still complicate the analysis and the high C—F absorbance of the PTFE substrate in the region of 1300 to 450 cm−1 and the high aliphatic C—H stretching absorbance of the polyethylene substrate in the 2918-2849 cm−1 region makes both cards of somewhat limited utility because running a background with such strong absorbances does not completely ratio out the absorbance peaks. Furthermore, taking backgrounds to compensate for spurious absorbance peaks in the sampling substrate requires a time consuming extra step which may defeat the purpose of using a disposable sample card for a quick qualitative analysis.
Another company, Janos Technology, Inc., currently markets a sample card which uses a screen or mesh as the sampling substrate of the type disclosed in U.S. Pat. Nos. 5,453,252 & 5,723,341. Although mesh cards do not create absorbance peaks, they do suffer from several deficiencies as sampling substrates. First, the mesh does not retain liquid samples very well in the vertical position. The aforesaid patents state that using the preferred mesh size for the screen sampling substrate will result in a contiguous film for about 10 seconds, which is too short a time to run more than a few scans on an FTIR spectrophotometer and is too short to complete even a single scan on a plotting dispersible IR spectrophotometer. In addition, there are frequently voids in mesh card samples. Furthermore, liquid samples applied to mesh form a meniscus at the interface of the mesh lattice with the liquid which makes the sample thicker in some places than it is in others. Many liquid samples analyzed by IR or FTIR spectroscopy are solutions which reach a viscoelastic state and then solidify into films. It is often preferable to allow the solvent in such a sample to evaporate, leaving a dry sample film of the solute. The remaining dry cured film is then analyzed by placing it in the beam of the spectrophotometer either on a transmissive substrate or as a free standing film. This method eliminates the complication of analyzing both the solvent and the solute. If the surface tension of the film on a screen or mesh sampling substrate is overcome by gravity within 10 seconds, many films will break before they can solidify. Similarly, mesh sampling substrates are not well suited for analysis of weakly absorbing solutions which do not reach a viscoelastic state, as when the solvent is evaporated the remaining sample is not dispersed evenly over the sampling supporting substrate. Another shortcoming of mesh sample support substrates is that when the preferred mesh size is used, only 50% of the energy available from the spectrophotometer is transmitted. This is sufficient for analysis of many samples, but many spectroscopists prefer more efficient energy throughput and prefer not to have to run a background to adjust for the lower energy throughput simply to make the spectra obtained more readable. Backgrounds take time to run and they cannot be run on all spectroscopic analytical instruments.
Spectroscopists have, for many years, used polished crystals and unpolished crystal blanks to perform qualitative and quantitative analysis of liquid and solid samples with IR and FTIR spectroscopic analytical instruments. Polished crystal windows are expensive and are, therefore, not at all comparable to disposable sample cards with polymer or mesh sample supporting windows. These precision polished windows are typically used within cells for quantitative and qualitative analysis.
On the other hand, unpolished crystal windows, known as “blanks”, are relatively inexpensive and are used for the same purpose as IR sample cards. Crystal blank windows used as sampling substrates are typically made from alkali halide crystals, such as KBr, NaCl and KCl. An unpolished alkali halide window will not exhibit any spurious absorbance peaks when the infrared beam of a spectrophotometer is passed through the window and the instrument is set at normal detection limits. Energy transmission of halide blanks is enhanced by a quick water polish which can be done by the spectroscopist with minimal equipment.
As can be seen in FIGS. 3A, 3B, 4A, 4B, 5A, and 5B, the transmission of these materials is enhanced measurably by water polishing. Neither training as an optician nor special equipment are required to water polish NaCl or KCl blanks. They can be effectively water polished by rubbing them on a paper towel on which a water and alcohol solution has been deposited on part of the towel. The crystal blank is simply rubbed alternately on the wet area of the towel and then back onto the dry area until adequate transparency is achieved. KBr blanks are water polished using the same technique, but the blanks must first be conditioned on a soft optical polishing cloth with the proper polishing compound. The polishing cloths and compounds are sold in inexpensive polishing kits that can be obtained from vendors of spectroscopy optics. A water polished KCl blank will transmit well in excess of 90% of the of the available energy over the range of 4400 cm−1 to 500 cm−1 (See FIG. 3B), while water polished NaCl and KBr blanks transmit in excess of 85% of the available energy in the ranges, respectively, of 4400 cm−1 to 666 cm−1 and 4400 cm−1 to 450 cm−1 (See FIGS. 4B & 5B).
Crystal blanks make more desirable sample supporting windows than sample cards using PTFE or polyethylene as windows, because the crystal windows have transmission and absorbance properties superior to polymers. Furthermore, windows made from crystal blanks can be used in pairs to create sandwich cells and can be used to efficiently cast films from solutions. Unfortunately, crystal blanks cost 2.5 to 4 times what a polymer sample card will cost and the crystal blank windows also require use of a holder to orient them in the spectrophotometer beam. The number of samples that can be scanned on a water polished crystal blank far exceeds the single sample scan that is taken with a disposable sample card made with mesh or polymer windows, because it is possible to repolish crystal blanks. Spectroscopists have repolished crystal windows for years, however, many labs now consider the labor cost of repolishing a crystal blank window to be too high to justify the effort.
To process an alkali halide crystal into a blank requires several operations in an optical shop. First, a crystal boule must be grown in a furnace. The boule is tested after a growth cycle spanning several days to determine if the requisite optical purity has been achieved. If the boule passes the purity test, it is then annealed. An external dimension must then be created. The external dimension is usually dictated by the size and type of holder in which the optic will be used. The holder is required to orient the [optic] window as a sample supporting substrate in the beam of a spectrophotometer. In the case of a circular crystal window or disc, for example, the external dimension is the diameter and the process of shaping the diameter begins by core drilling the crystal boule and then centerless grinding the rod into its final diameter. Individual disc shaped pieces are then cut from the rod. An alternative is sawing or cleaving the material into individual pieces and then edging the outside diameter of each piece. The faces of the disc are then lapped in stages on optical lapping machines using grinding compounds with progressively smaller particles. The lapping process causes the crystal to become opaque when it is ground with courser grinding compounds. Unless the opacity is overcome by a fine polish using smaller size grinding compounds or by a water polish (as discussed earlier), the opacity of the optic dramatically limits the ability of the optic to transmit light energy (including the infrared energy used in an IR or FTIR spectrophotometer). At each stage the optics are cleaned; first, to keep larger particulate grinding compound from contaminating smaller particulate stages of the process and next, to remove the finest grinding compound and chemicals in the grinding compound carrier slurry. This multi-step process is labor intensive and expensive, which is one of the reasons that crystal blanks are more expensive sample supporting windows than polymers or screens and mesh.
For many years infrared spectroscopists have pressed windows known as “pellets” of a matrix of KBr and a solid sample to produce spectra of the solid sample. Unfortunately, production of pressed windows from powders requires a number of labor intensive steps and the powders require careful preparation and handling. Contamination of the windows during the pressing process is difficult to avoid when mass production is contemplated. As such, although an adequate window can be pressed from powders, only a few materials which are capable of transmitting infrared energy, such as silver chloride and zinc sulfide, are well suited to pressing. These materials are relatively expensive. Cheaper materials, such as KBr and NaCl, can be pressed into ½′ diameter pellets with some success, but as the pressed window diameter gets larger the success rate drops. Pressed windows that are in the 20 to 25 mm diameter range require high pressure, laborious preparation and elaborate precautions against contamination. The resulting windows rapidly absorb moisture which causes them to fog or haze over and they quickly reach the point where energy transmission becomes marginal. Producing a low cost sample supporting window for use in a sample card using this technology is therefore simply not practical.
The use of sample cards for spectroscopic sampling is known in the art. As noted above, 3M produced sample cards which utilized PTFE and polyethylene windows as sample support substrates but it has since discontinued the manufacture of the cards and other vendors have begun to manufacture them. The polymer sample supporting windows do, however, exhibit the undesirable absorbance peaks referenced above which limit their utility. Another similar device is the mesh or screen card in which a liquid sample adheres to the mesh and the beam of the spectrophotometer can pass through the free standing sample. The screen cards exhibit less than optimal energy transmission, the length of time that they will support liquid samples is quite short, and they do not provide a useful platform for sampling solutions. Neither type card can be used to sandwich a sample. Both sample cards are inferior in performance to unpolished crystal blanks. But, both of these sample cards also have the advantage of being inexpensive enough to be disposable and of providing their own sample holder in the form of a cardboard frame for mounting and orienting the sample in the beam of the spectrophotometer on which the spectroscopist can write information regarding the sample as shown in FIG. 7, which makes them convenient to use in the lab.
It would be an advance in the art to provide a sampling device with the convenience and low cost of the polymer window and screen sample cards, but with the superior optical and physical properties of water polished crystal blank windows.